Goal:
  
  A : Type
  B : Type
  R : nat → A → B → Type
  n : nat
  x : A
  y : B
  l : list A
  r : list B
  r0 : R n x y
  a : All2i R (S n) l r
  b : All2i R n (x :: l) (y :: r)
  ============================
   (NoConfusionHom_All2i A B R n (x :: l) (y :: r) (All2i_cons r0 a) b
    → let H := CoreTactics.block in All2i_cons r0 a = b)


DEBUG:Going to execute:
match goal with
           | |- ?T => change (let _ := CoreTactics.block in T)
           end